Reclaimed elastomer materials (i.e., “reclaimed materials,” “ground tire rubber,” “GTR,” “micronized rubber powders,” or “MRP”), which include vulcanized elastomer materials, are used in a variety of applications, including elastomer compositions (e.g., tire tread compounds for vehicle tires), plastics compositions (e.g., as fillers for polyolefins), asphalt fillers, and others. In many of these applications, the micronized rubber powders are used as “filler” in place of a portion of the virgin compound material. One of the primary reasons for the use of reclaimed elastomer materials is cost. Moreover, rubber powders, whether GTR or MRP, are typically significantly less expensive than virgin (i.e., non-reclaimed) rubber or plastic, and when used as a “filler” in elastomer or plastic compositions, tends to reduce the overall manufacturing cost of the composition. Further, because micronized rubber powders typically are made from recycled or reclaimed material (e.g., vulcanized scrap from manufacturing processes and used tires or other elastomeric products), reincorporating them into elastomer and plastic compositions reduces landfill waste and results in a more environmentally-friendly product. Finally, use of recycled GTR or MRP provides a strategic supply chain hedge against petroleum-based supply chain price and supply volatility.
Generally, GTR consists of particle size distributions that range from a diameter of 2 mm to 0.5 mm and are produced in a variety of ways including grinding methods such as cryogenic grinding. Micronized rubber powder, or MRP, is termed as such because it generally contains a significant fraction of particles less than 100 microns in size. MRP may also be produced by either ambient or cryogenic grinding methods. Powders such as GTR and MRP are commonly designated by their mesh size. For example, powders in the size range of 10-30 mesh generally are considered GTR, while 40-300 mesh materials generally are considered MRP. GTR typically is less expensive than MRP because of the higher manufacturing cost required for making the smaller particles. Because of this cost difference, GTR generally is used instead of MRP, unless the specific application requires properties than can only be achieved through the use of MRP. As noted previously, reclaimed elastomer materials used to manufacture elastomer particles generally are obtained from previously-manufactured products (e.g., used and unused vehicle tires) and from vulcanized scrap generated during the manufacturing process (e.g., vulcanized scrap generated from the tire manufacturing process), and thus the reclaimed elastomer material is generally vulcanized.
Vulcanized particles are relatively inert, i.e., the particles are non-reactive with virgin matrix materials. As such, they are limited in their use as a component in elastomer compositions because when added at high levels, the resultant elastomer composition exhibits diminished performance characteristics. This limitation, however, can be eased by the use of functionalized particles.
Vulcanized elastomer material can be functionalized through a variety of processes. Broadly speaking, functionalization involves modifying the chemistry of the vulcanized particles. One such functionalization process is devulcanization. Devulcanization processes are disclosed, for example, in U.S. Pat. No. 5,770,632, U.S. Pat. No. 6,451,526 B1, U.S. Pat. No. 6,831,109 B1, European Patent Application No. 0 748 837 A1, European Patent Application No. 1 242 520 B1, European Patent Application No. 0 690 091 A1 and U.S. Patent Application Publication No. 2010/0317752. Generally speaking, devulcanization involves the chemical, thermal, and/or mechanical treatment of vulcanized elastomer to break the chemical crosslinks formed during the vulcanization process. One devulcanization process involves applying a chemical additive to the reclaimed vulcanized particles while the particles are under shear stresses. This type of functionalization is performed, for example, by Levgum, Ltd., having a principal place of business in Kanot, Israel. An alternate devulcanization process utilizes high temperatures as opposed to chemicals to break the chemical crosslinks. In these processes, the input powder is reacted and generally converted into a spongy mass. The resulting devulcanized (i.e., functionalized) material can then be used in admixtures with virgin elastomeric materials. Previously, in terms of reclaimed rubber powders, only GTR has been considered as a raw material for functionalization, for three reasons. First, the GTR is lower in cost. Second, the resultant “spongy mass” functionalized material is identical in appearance regardless of the particle size of the input material. Third, the assumption has been that, in a chemical, thermal and/or mechanical devulcanization process, the input particle size is irrelevant since the entire mass of the material is converted during the devulcanization process. For example, in U.S. Pat. No. 6,831,109, vulcanized rubber in the form of crumb having a size of 0.5 to 5.0 mm or pieces having a thickness of up to 15 mm was devulcanized in a two roll mill.
While elastomer compositions comprising functionalized GTR exhibit improved mechanical properties as compared to elastomer compositions comprising vulcanized GTR, the use of functionalized GTR in admixture with virgin material can still result in a reduction in the mechanical properties of the resulting elastomer formulations. It was assumed previously that elastomer compositions comprising functionalized GTR or functionalized MRP would exhibit comparable performance characteristics irrespective of the size of the input functionalized particles. Specifically, the conventional assumption has been that elastomer compositions and other material compositions comprising functionalized rubber powder of any particle size would exhibit diminished strength and durability properties (e.g., measured through tensile strength tests) as compared to those comprising virgin elastomer materials. Further, it has been assumed that the functionalization process renders the particle size of the input material irrelevant with respect to the final properties of admixed compositions.
Therefore, there is a long-felt but unresolved need for elastomer compositions that comprise reclaimed elastomer materials, but which retain mechanical properties comparable to elastomer compositions containing no reclaimed material. The use of functionalized MRP can meet this need.